Air turbine starter with decoupler

ABSTRACT

A method and decoupler for disengaging an output shaft from an engine in a back drive event with a backdrive decoupler. The backdrive decoupler includes a an output shaft, drive shaft, and a retention mechanism selectively coupling the output shaft to the drive shaft. In a backdrive event, the decoupler decouples the output shaft from the drive shaft.

BACKGROUND OF THE INVENTION

A driving mechanism, such as a motor or engine, can generate drivingmotions at a mechanism output, such as at a rotatable output shaft. Theoutput shaft can, for example, provide a rotational kinetic motion toanother piece of equipment via a rotatable drive shaft connected to theoutput shaft. The piece of equipment receiving the rotational kineticmotion can utilize the driving rotational motion as an energy source tooperate. In one example configuration, a gas turbine engine, also knownas a combustion turbine engine, is a rotary engine that extracts energyfrom a flow of combusted gases passing through the engine onto amultitude of turbine blades. The gas turbine engine can provide at leasta portion of the rotational kinetic motion to rotating equipment, suchas an accessory gearbox, where the rotational motion is utilized topower a number of different accessories. The accessories can includegenerators, starter/generators, permanent magnet alternators (PMA) orpermanent magnet generators (PMG), fuel pumps, and hydraulic pumps. Inthe event of failure of the driving mechanism, it can be desirable todecouple the driving mechanism from the rotating equipment.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, the present disclosure relates to an air turbine starterfor starting an engine, including a housing defining an inlet, anoutlet, and a flow path extending between the inlet and the outlet forcommunicating a flow of gas there through, a turbine member journaledwithin the housing and disposed within the flow path for rotatablyextracting mechanical power from the flow of gas, a gear train drivinglycoupled with the turbine member, a clutch having a drive shaft operablycoupled with the gear train; and a decoupler, including: a tensile fusehaving a first end operably coupled to the drive shaft, a threadedportion, and a neck portion having a reduced diameter located betweenthe first end and the threaded portion; and an output shaft having afirst end selectively operably coupled to the drive shaft, a second endconfigured to be operably coupled to and rotate with the engine, and aninternal threaded portion that receives the threaded portion of thetensile fuse; wherein when driving torque is transmitted from the driveshaft of the clutch to the output shaft the tensile fuse is not loaded,when overrunning torque is transmitted below a certain level the tensilefuse is partially loaded and when the overrunning torque reaches acertain level the tensile fuse shears at the neck portion and thethreaded portion is threaded in a direction away from the drive shaft.

In another aspect, the present disclosure relates to a decouplerassembly for decoupling an output shaft of an air turbine starter duringbackdrive, including: a tensile fuse having a first end operably coupledto a drive shaft of the air turbine starter, a threaded portionreceivable within an internal threaded portion of the output shaft ofthe air turbine starter, and a neck portion located between the firstend and the threaded portion; and wherein when driving torque istransmitted from the drive shaft to the output shaft the tensile fuse isnot loaded, when overrunning torque is transmitted below a certain levelthe tensile fuse is partially loaded and when the overrunning torquereaches a certain level the tensile fuse shears at the neck portion andthe threaded portion is threaded in a direction away from the driveshaft.

In yet another aspect, the present disclosure relates to a method foroperating an air turbine starter, including: extracting mechanical powerfrom a flow of gas utilizing a turbine and driving a gear train andclutch having a drive shaft therewith; transmitting a driving torquefrom the drive shaft to an output shaft operably coupled to an engine;and during back driving, activating a backdrive decoupler wherein atensile fuse operably coupled to the output shaft and the drive shaft issheared and a sheared portion of the tensile fuse is unwound from theoutput shaft and translated away from the drive shaft and the outputshaft is unwound from the drive shaft and translated away from the driveshaft.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic illustration of a gas turbine engine with anaccessory gearbox in accordance with various aspects described herein.

FIG. 2 is a sectional view of a starter/generator mounted to theaccessory gearbox of FIG. 1 by way of a rotating shaft, in accordancewith various aspects described herein.

FIG. 3 is an exploded view of a decoupler adapted for use with thestarter/generator of FIG. 2 in accordance with various aspects describedherein.

FIG. 4A is a cross-sectional view of the decoupler of FIG. 3 in a firstposition relative to the rotating shaft, in accordance with variousaspects described herein.

FIG. 4B is a top view of the decoupler of FIG. 3 in a first positionrelative to the rotating shaft, in accordance with various aspectsdescribed herein.

FIG. 5A is a cross-sectional view of the decoupler of FIG. 3 in a secondposition relative to the rotating shaft, in accordance with variousaspects described herein.

FIG. 5B is a top view of the decoupler of FIG. 3 in a second positionrelative to the rotating shaft, in accordance with various aspectsdescribed herein.

FIG. 6 is a flow chart of a method for operating an air turbine starter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is related to a driving mechanism generatingkinetic motion in the form of a rotating shaft coupled with a piece ofrotating equipment. One non-limiting example of a driving mechanism caninclude a gas turbine engine rotationally driving a piece of rotatingequipment, such as a starter. The starter has various applicationsincluding starting a gas turbine engine and generating electrical powerwhen the gas turbine engine is in operation. While the exemplaryembodiment described herein is directed to application of a gas turbineengine and a starter, embodiments of the disclosure can be applied toany implementation of a driving mechanism that generates rotationalmotion at a driving output, and provides the rotational motion toanother piece of rotating equipment.

Referring to FIG. 1, a starter motor or air turbine starter 102including an accessory gear box (AGB) 100, also known as a transmissionhousing, are schematically illustrated as being mounted to a gas turbineengine 1. This assembly is commonly referred to as an IntegratedStarter/Generator Gearbox (ISGB), or simply an air turbine starter 102.The gas turbine engine 1 comprises an air intake with a fan 50 thatsupplies air to a high pressure compression region 60. The air intakewith a fan 50 and the high pressure compression region collectively areknown as the ‘cold section’ of the gas turbine engine upstream of thecombustion. The high pressure compression region 60 provides thecombustion chamber 10 with high pressure air. In the combustion chamber,the high pressure air is mixed with fuel and combusted. The hot andpressurized combusted gas passes through a high pressure turbine region20 and a low pressure turbine region 30 before exhausting from the gasturbine engine. As the pressurized gases pass through the high pressureturbine (not shown) of the high pressure turbine region 20 and the lowpressure turbine (not shown) of the low pressure turbine region 30, theturbines extract rotational energy from the flow of the gases passingthrough the gas turbine engine 1. The high pressure turbine of the highpressure turbine region 20 can be coupled to the compression mechanism(not shown) of the high pressure compression region 60 by way of a shaftto power the compression mechanism. The low pressure turbine can becoupled to the fan 50 of the air intake by way of a shaft to power thefan 50.

The gas turbine engine can be a turbofan engine, such as a GeneralElectric GEnx or CF6 series engine, commonly used in modern commercialand military aviation or it could be a variety of other known gasturbine engines such as a turboprop or turboshaft. The gas turbineengine can also have an afterburner that burns an additional amount offuel downstream of the low pressure turbine region 30 to increase thevelocity of the exhausted gases, and thereby increasing thrust.

The AGB 100 is coupled to a turbine shaft of the gas turbine engine 1,either to the low pressure or high pressure turbine by way of amechanical power take-off 90. The mechanical power take off 90 containsmultiple gears and means for mechanical coupling of the AGB 100 to thegas turbine engine 1. The assembly 102 can be mounted on the outside ofeither the air intake region containing the fan 50 or on the core nearthe high pressure compression region 60.

Referring now to FIG. 2, the air turbine starter 102 is shown in greaterdetail. Generally, the air turbine starter 102 includes a housing 108defining an inlet 110, an outlet 112, and a flow path 114 extendingbetween the inlet 110 and outlet 112 for communicating a flow of gasthere through. The air turbine starter 102 includes a turbine member 116journaled within the housing 108 and disposed within the flow path 114for rotatably extracting mechanical power from the flow of gas along theflow path 114. Further, a gear train 118, disposed within the gear box101 and drivingly coupled with the turbine member 116, can be caused torotate.

The gear train 118 includes a ring gear 120 and can further comprise anygear assembly including for example but not limited to a planetary gearassembly or a pinion gear assembly. A turbine shaft 122 couples the geartrain 118 to the turbine member 116 allowing for the transfer ofmechanical power. The turbine shaft 122 is rotatably mounted to the geartrain 118 and supported by a pair of turbine bearings 124 while the geartrain 118 is supported by a pair of carrier bearings 126.

A gear box interior 127 can contain oil to provide lubrication andcooling to mechanical parts contained therein such as the gear train118, ring gear 120, and bearings 124, 126.

There is an aperture 128 through which the turbine shaft 122 extends andmeshes with a carrier shaft 130 to which a clutch 132 is mounted andsupported by a pair of spaced bearings 134. A drive shaft 136 extendsfrom a portion of the gear box 101 and is coupled to the clutch 132 andadditionally supported by the pair of spaced bearings 134. The driveshaft 136 is driven by the gear train 118 and coupled to the powertake-off 90 of the gas turbine engine 1, such that operation of theengine 1 provides a driving motion to the gear box 101.

The clutch 132 can be any type of shaft interface portion that forms asingle rotatable shaft 138 including the turbine shaft 122, the carriershaft 130, and the drive shaft 136. The shaft interface portion can beby any known method of coupling including, but not limited to, gears,splines, a clutch mechanism, or combinations thereof. An example of ashaft interface portion 132 is disclosed in U.S. Pat. No. 4,281,942 toGeneral Electric and is incorporated herein by reference in itsentirety.

The gear box 101 and the starter 102 can be formed by any knownmaterials and methods, including, but not limited to, die-casting ofhigh strength and lightweight metals such as aluminum, stainless steel,iron, or titanium. The housing for the gear box 101 and starter 102 canbe formed with a thickness sufficient to provide adequate mechanicalrigidity without adding unnecessary weight to the assembly 102 and,therefore, the aircraft.

The rotatable shaft 138 can be constructed by any known materials andmethods, including, but not limited to extrusion or machining of highstrength metal alloys such as those containing aluminum, iron, nickel,chromium, titanium, tungsten, vanadium, or molybdenum. The diameter ofthe turbine shaft 122, carrier shaft 130, and drive shaft 136 can befixed or vary along the length of the rotatable shaft 138. The diametercan vary to accommodate different sizes, as well as rotor to statorspacing.

As described herein, either the gear box 101 or the starter 102 can be adriving mechanism for driving the rotation of the rotating shafts 122,130, 136. For example, during starting operations, the starter 102 canbe the driving mechanism for rotation of the rotating shafts 122, 130,136. Alternatively, during normal gas turbine engine 1 operation, thegear box 101 can be the driving mechanism for rotation of the rotatingshafts 122, 130, 136. The non-driving mechanism, that is, the equipmentbeing driven by the driving mechanism, can be understood as rotatingequipment utilizing the rotational movement of the rotating shafts 122,130, 136, for example to generate electricity in the starter 102.

The drive shaft 136 is further coupled to a decoupler assembly 190including a backdrive decoupler 200 having an output shaft 202,configured to be operably coupled to and rotate with the engine 1, and atensile fuse 204. The tensile fuse 204 is selectively receivable andaxially moveable within an internal threaded portion 206, that can befor example a helical female thread, of the output shaft 202. Alljoining parts can be formed from steel or like materials.

FIG. 3 illustrates an exploded perspective view of one exemplaryembodiment of the decoupler assembly 190 including the backdrivedecoupler 200 and the drive shaft 136. The drive shaft 136 can include acircular depression 210 and an orthogonal depressions 212. The circulardepression 210 is surrounded by the orthogonal depression 212. A centralopening 214 is bound by a circular lip 216. Circumscribing the circularlip 216 are a plurality of stops 218.

A thread insert 208 can be included in the decoupler assembly 190 and isillustrated as including a complementary circular protrusion 220 andorthogonal portion 222 formed to fit into the circular and orthogonalshaped depressions 210, 212 of the drive shaft 136. The thread insert208 further includes an internal helical threaded portion 224 which canbe for example but not limited to a three helical female thread.

The decoupler assembly 190 can also include a compression spring 217 anda ring spring 228.

The output shaft 202 includes a first end 230 having an exteriorlythreaded portion 234. The exteriorly threaded portion 234 can include,but is not limited to, a helical male thread, with a proximal endincluding complementary stops 236. A second end 232 is configured to beoperably coupled to and rotate with the engine 1.

The tensile fuse 204 includes a cylindrical first end 238. A second end240 of the tensile fuse 204 includes a head 242 and a threaded portion244 that can be, for example, a helical male thread, disposed beneaththe head 242. The tensile fuse 204 further includes a neck portion 246having a reduced diameter between the first end 238 and the threadedportion 244.

The cylindrical first end 238 is received in the central opening 214where a retainer pin 239 retains the first end 238 of the tensile fuse204 within the drive shaft 136 operably coupling the tensile fuse 204 tothe drive shaft 136. At assembly the tensile fuse 204 can be drilled andpinned with the retainer pin 239. The threaded portion 244 disposedbeneath the head 242 is received within the internal threaded portion206 of the output shaft 202

The exteriorly threaded portion 234 of the output shaft 202 and theinternal threaded portion 206 of the tensile fuse 204 are formed so thatthe tensile fuse 204 is threaded into the output shaft 202 with anopposite hand turn compared to when the output shaft 202 is threadedinto the thread insert 208. The stops 218 and complementary stops 236decrease axial loads along the threaded portions, 206, 224, 234, 244when the output shaft 202 is fully threaded into the thread insert 208of the drive shaft 136.

FIG. 4A illustrates the decoupler assembly 190 mounted with the driveshaft 136. Because the drive shaft 136 and the thread insert 208 havecorresponding circular depression 210, orthogonal depressions 212 andcircular protrusion 220 and orthogonal portions 222, respectively, theycan be operably coupled. Likewise, the complementary shapes of the stop218 and 236 operably couple the drive shaft 136 to the output shaft 202.It should be understood that the shapes of the depressions andcorresponding receiving portions depicted as orthogonal and circular andthe shapes of the complementary stops are for illustrative purposes andnot meant to be limiting.

A biasing mechanism can be included between the thread insert 208 andthe drive shaft 136. In the illustrated example, the biasing mechanismis the compression spring 217.

A load path can go through mating stop features including the stops 218,236 and transmit a driving torque. Under normal operating conditions thedriving torque is transmitted from the drive shaft 136 of the clutch 132to the output shaft 202 to drive the engine 1 by the mating stopfeatures 218, 236. The load path leaves the threaded portions, 206, 224,234, 244, the tensile fuse 204, and the retainer pin 239 unloaded.

A top view of the thread insert 208 in FIG. 4B illustrates the ringspring 228. Under normal operating conditions the threads of theexteriorly threaded portion 234 push the ring spring 228 outward into aninner portion 226 of the thread insert 208 in an expanded position.

When the clutch 132 becomes disengaged and the engine 1 transmits anoverrunning torque, having a magnitude below a certain level, to the airturbine starter 102 the mating stop features 218, 236 become unloadedwhile the threaded portions, 206, 224, 234, 244, the tensile fuse 204,and retainer pin 239 become partially loaded.

Turning to FIG. 5A, in the event of a backdrive which can occur when theclutch 132 fails, the air turbine starter 102 decouples its load path sothat components of the gear box 101 and starter 102 are disconnectedfrom the engine 1. The failing clutch 132 becomes engaged or lockedwhile the engine 1 transmits an overrunning torque, with a magnitude oftorque at or above the certain level, to the air turbine starter 102.This can also be considered a back driving torque.

In the case of the locked clutch, the backdrive decoupler 200 would beexposed to enough drag torque that the output shaft 202 would unwindfrom the drive shaft 136, and to simultaneously unwind the tensile fuse204 from the output shaft 202. The thread ratios between the internalthreaded portion 206 and the threaded portion 244 for the tensile fuse204 compared to those between the internal threaded portion 224 and thethreaded portion 234 for the output shaft 202 allow for the tensile fuse204 to translate two times the translation distance of the output shaft202, contributing to a high strain on the neck portion 246, causing itto shear and creating a sheared portion 250 and a base 252. The shearedportion 250 of the tensile fuse 204 is unwound from the output shaft 202leaving the base 252 retained by the retainer pin 239.

The unwinding of the output shaft 202 is further aided by thecompressive spring 217. When the output shaft 202 begins to unwind, thecompressive spring 217 is configured to expand and bias the output shaft202 away from the drive shaft 136. It will be understood that anysuitable biasing mechanism can be utilized and that the compressivespring is one illustrated example.

A top view of the thread insert 208 in FIG. 5B illustrates the movementof the ring spring 228 in the event of a back drive. Because the outputshaft 202 has translated, the threads of the exteriorly threaded portion234 are no longer pushing the ring spring 228 outward. This allows thering spring 228 to compress to a retracted position where the ringspring 228 overlies a portion of the thread insert 208 preventingrethreading of the first end 230 of the output shaft 202 and the threadinsert 208. In this manner, the ring spring 228 is configured to securethe output shaft 202 in a decoupled and separate position from the driveshaft 136.

A method 400 for operating an air turbine starter 102 is outlined in aflow chart in FIG. 6. The method 400 begins at 402 with extractingmechanical power from a flow of gas utilizing a turbine 116 and drivingthe gear train 118 and clutch 132, including the drive shaft 136. At 404a driving torque is transmitted from the drive shaft 136 to an outputshaft 202 which is operably coupled to the engine 1.

In the case of back driving at 406 the backdrive decoupler 200 isactivated when the tensile fuse 204 that is operably coupled to both theoutput shaft 202 and the drive shaft 136 is sheared. The sheared portion250 of the tensile fuse 204 is then unwound from the output shaft 202and translated away from the drive shaft 136. The output shaft 202 isunwound from the drive shaft 136 and translated away from the driveshaft 136.

At 408 the output shaft 202 is prevented from reengaging the drive shaft136 when the ring spring 228 contracts. The contraction of the ringspring 228 prevents the output shaft 202 from reengaging by blocking theinternal helical threaded portion 224 from receiving the exteriorlythreaded portion 234. The compression spring 217 is a secondarymechanism that also prevents reengagement when it has sprung. Thespringing of the compression spring 217 biases the output shaft202 outand away from the drive shaft 136. The air turbine starter 102 istherefore disabled after decoupling, which prevents an additional enginestart.

All directional references (e.g., radial, upper, lower, upward,downward, left, right, lateral, front, back, top, bottom, above, below,vertical, horizontal, clockwise, counterclockwise) are only used foridentification purposes to aid the reader's understanding of thedisclosure, and do not create limitations, particularly as to theposition, orientation, or use thereof. Connection references (e.g.,attached, coupled, connected, and joined) are to be construed broadlyand can include intermediate members between a collection of elementsand relative movement between elements unless otherwise indicated. Assuch, connection references do not necessarily infer that two elementsare directly connected and in fixed relation to each other. Theexemplary drawings are for purposes of illustration only and thedimensions, positions, order and relative sizes reflected in thedrawings attached hereto can vary.

Many other possible embodiments and configurations in addition to thatshown in the above figures are contemplated by the present disclosure.Additionally, the design and placement of the various components such asstarter, AGB, or components thereof can be rearranged such that a numberof different in-line configurations could be realized.

The aspects of the present disclosure provide a decoupler for decouplinga torque load coming from the gear train of an engine to preventbackdriving of the entire air turbine starter. Benefits associated withthis decoupling include reducing the risk of spinning a damaged airturbine starter which could cause additional damage to the air turbinestarter if not decoupled. Further still, the decoupling results in onlythe tensile fuse being needed to be replaced instead of more costlyparts damaged by the continued backdriving.

To the extent not already described, the different features andstructures of the various embodiments can be used in combination witheach other as desired. That one feature cannot be illustrated in all ofthe embodiments is not meant to be construed that it cannot be, but isdone for brevity of description. Thus, the various features of thedifferent embodiments can be mixed and matched as desired to form newembodiments, whether or not the new embodiments are expressly described.Moreover, while “a set of” various elements have been described, it willbe understood that “a set” can include any number of the respectiveelements, including only one element. Combinations or permutations offeatures described herein are covered by this disclosure.

This written description uses examples to disclose embodiments of theinvention, including the best mode, and also to enable any personskilled in the art to practice embodiments of the invention, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the invention is defined by the claims,and can include other examples that occur to those skilled in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

1. A decoupler assembly for decoupling an output shaft of a startermotor during backdrive, comprising: a tensile fuse having a first endoperably coupled to a drive shaft of the starter motor, a threadedportion receivable within an internal threaded portion of the outputshaft of the starter motor, and a neck portion located between the firstend and the threaded portion; and wherein when a driving torque istransmitted from the drive shaft to the output shaft the tensile fuse isnot loaded, when overrunning torque is transmitted below a certain levelthe tensile fuse is partially loaded and when the overrunning torquereaches a certain level the tensile fuse shears at the neck portion andthe threaded portion is threaded in a direction away from the driveshaft.
 2. The decoupler of claim 1, further comprising a thread insertoperably coupled to the drive shaft and having a threaded portion thatreceives a threaded portion of the output shaft of the starter motor. 3.The decoupler of claim 2 wherein the tensile fuse is threaded into theoutput shaft with an opposite hand turn to the threads between theoutput shaft and the thread insert.
 4. The decoupler of claim 3 whereinwhen the overrunning torque reaches the certain level the output shaftis unthreaded from the thread insert such that it is uncoupled from thedrive shaft and the tensile fuse is unthreaded from the output shaft. 5.The decoupler of claim 4 wherein the tensile fuse translates two timesthe translation of the output shaft.
 6. The decoupler of claim 4,further comprising a blocking mechanism configured to bar the outputshaft from rethreading with the thread insert.
 7. An air turbine starterfor starting an engine, comprising: a housing defining an inlet, anoutlet, and a flow path extending between the inlet and the outlet forcommunicating a flow of gas therethrough; a turbine member journaledwithin the housing and disposed within the flow path for rotatablyextracting mechanical power from the flow of gas; a gear train drivinglycoupled with the turbine member; a clutch having a drive shaft operablycoupled with the gear train; and a decoupler, comprising: a tensile fusehaving a first end operably coupled to the drive shaft, a threadedportion, and a neck portion having a reduced diameter located betweenthe first end and the threaded portion; and an output shaft having afirst end selectively operably coupled to the drive shaft, a second endconfigured to be operably coupled to and rotate with the engine, and aninternal threaded portion that receives the threaded portion of thetensile fuse; and wherein when a driving torque is transmitted from thedrive shaft of the clutch to the output shaft the tensile fuse is notloaded, when an overrunning torque is transmitted below a certain levelthe tensile fuse is partially loaded and when the overrunning torquereaches a certain level the tensile fuse shears at the neck portion andthe threaded portion is threaded in a direction away from the driveshaft.
 8. The air turbine starter of claim 7, further comprising athread insert operably coupled to the drive shaft and having a threadedportion that receives an exteriorly threaded portion of the first end ofthe output shaft.
 9. The air turbine starter of claim 8 wherein thetensile fuse is threaded into the output shaft with an opposite handturn to the threads between the output shaft and the thread insert. 10.The air turbine starter of claim 9 wherein when the overrunning torquereaches the certain level the output shaft is unthreaded from the threadinsert such that it is uncoupled from the drive shaft and the tensilefuse is unthreaded from the output shaft.
 11. The air turbine starter ofclaim 10 wherein the tensile fuse translates two times the translationof the output shaft.
 12. The air turbine starter of claim 10, furthercomprising a blocking mechanism configured to bar the output shaft fromrethreading with the thread insert.
 13. The air turbine starter of claim12 wherein the blocking mechanism comprises a ring spring moveablebetween an expanded position where the ring spring is pushed outwards bythe output shaft and a retracted position where the ring spring overliesa portion of the thread insert and is configured to prevent rethreadingof the first end of the output shaft and the thread insert.
 14. The airturbine of claim 10, further comprising a biasing mechanism locatedbetween the drive shaft and the output shaft and configured to bias theoutput shaft away from the drive shaft.
 15. The air turbine of claim 14wherein the biasing mechanism is a compressive spring.
 16. The airturbine starter of claim 13, further comprising a retainer pin retainingthe first end of the tensile fuse within the drive shaft.
 17. The airturbine starter of claim 13 wherein the driving torque is transmittedthrough mating stop features on the drive shaft and the output shaft.18. A method for operating a starter motor, comprising: extractingmechanical power from a flow of gas utilizing a turbine and driving agear train and clutch having a drive shaft therewith; transmitting adriving torque from the drive shaft to an output shaft operably coupledto an engine; and during back driving, activating a backdrive decouplerwherein a tensile fuse operably coupled to the output shaft and thedrive shaft is sheared and a sheared portion of the tensile fuse isunwound from the output shaft and translated away from the drive shaftand the output shaft is unwound from the drive shaft and translated awayfrom the drive shaft.
 19. The method of claim 18, wherein the tensilefuse translates two times the translation of the output shaft.
 20. Themethod of claim 18, further comprising preventing the output shaft fromreengaging the drive shaft such that the starter motor is disabled afterdecoupling, which prevents an additional engine start.